1. Field of the Invention
The present invention relates to servo control for a disk drive recording apparatus and more particularly to a method and apparatus for calibration of the gain for any hardware circuit components of the servo control system and to a disk drive recording apparatus having such a gain calibration function.
2. Description of the Related Art
In recent years a disk drive recording devices, and in particular hard disk drives, have been extensively used as among the most preferred data storage means for computer systems. The state of the art consequently has improved to offer higher speed for read/write operations as well as a greater data storage capacities. Accordingly, as data storage capacities for hard disk drives have increased in line with the rapid technical progress of personal computers, more precise servo control is generally necessitated to ensure operational reliability suitable for such disk drives to be used as data storage means.
A servo control system in the aforementioned hard disk drive has two principal operational modes: a seek mode and a track following mode. The seek mode is used for a read/write operation to relocate a head of the disk drive to a desired target track position, and the track following mode is used to have the head accurately follow a center course or a data line along the target track, once the head has reached the target track. Such a servo control system typically operates under velocity control until the head reaches the target track, and then it operates under position control at the target track so that the head is maintained on-track.
In the aforementioned hard disk drive, some hardware components of the servo control system, such as an analog-to-digital converter, an actuator such as a voice coil motor, an actuator driver, a digital-to-analog converter, and so forth, usually have their own offsets and gain differences. These offset and gain differences may be large or small, but it is known in the art that, when generated by such hardware elements, they have a significant influence upon the performance and stability of the entire servo control system. As will be discussed in detail below, this influence results because such gain differences, in particular, alter the velocity with which the actuator responds to a given velocity control command. Thus the actuator, in an initial or acceleration stage of a seek operation with a given stroke length, typically will attain a peak velocity different from the velocity contemplated in the design of the control system for a stroke of that length.
It is therefore appreciated that, at a given point in time within the accelerating phase, a velocity difference .increment.V is attributable to the gain difference .increment.K.sub.OP of the hardware circuit components of the servo control system. As a result, in a typical disk drive recording apparatus the gain difference of the hardware circuit components of the respective servo control system often degrades the servo control performance. In some cases this gain difference can even lead to malfunction of the servo control system.
Some techniques exist for calibration of disk drive servo control systems. For example, U.S. Pat. No. 4,480,217, entitled "Automatic Velocity Calibrator for a Velocity Servo Loop in a Magnetic Disk Drive" and granted to Robbins et al., shows a device for automatically calibrating actuator velocity in a disk drive. This system makes a series of measurements of the actuator velocity during normal seek operation of the disk drive, stores the measurements, and computes an average velocity value. This average approximates the instantaneous velocity of the actuator during one of these seek operations. The system then compares the average to the target velocity of a constant velocity command and increments or decrements a latch in accordance with the result. The latch value is then used to adjust the gain of the velocity transducer. This system therefore calibrates the actuator instantaneous velocity to a target velocity through a series of successive approximations.
Prior calibration approaches, such as that of Robbins et al., have certain benefits but do not provide a satisfactory level of true gain calibration. First, the Robbins system and similar systems calibrate velocity, not gain, and thus fail to recognize the important role that gain miscalibration can play in the actual performance characteristics of the disk drive. Second, the Robbins approach samples instantaneous velocity values during ongoing use of the disk drive and therefore does not take into account the substantial role that seek operations with relatively long stroke lengths play in performance degradation when the system has poorly calibrated gain. Third, these prior art systems tend to involve cumbersome computational schemes that fail to take advantage of the underlying system dynamics involved when gain miscalibration leads to performance degradation.
In view of the performance problems in servo control systems arising from miscalibration of hardware component gains and the limitations of existing calibration technology, I have found that a need exists for an improved gain calibration method and apparatus to be incorporated in high performance disk drives. Such an invention will reliably accomodate variations in hardware parameters between individual disk drive units, but it will also be capable of inexpensive and uniform implementation in a mass-produced disk drive product. Ideally, this gain calibration solution will be flexible to allow a wide variety of versions, yet sufficiently powerful to provide effective gain calibration in a wide range of hardware environments.